The invention relates to a photomultiplier tube heat sealed within an envelope, and particularly to a structure for shielding the dynodes of the cage assembly of such a tube from the deleterious effects of the heat generated during the sealing operation. The shielding structure also provides means for mounting at least one alkali vapor source within the envelope so that the alkali vapor source is also shielded from the heat generated during the sealing operation.
A photomultiplier tube for an application such as, for example, oil-well logging, is preferably small and rugged, since the tube must operate reliably in an environment where it is subjected to shock, vibration and high operating temperatures. Such a tube is described in my U.S. Pat. No. 4,355,258, filed Dec. 16, 1980, entitled, "PHOTOMULTIPLIER TUBE HAVING A STRESS ISOLATION CAGE ASSEMBLY," incorporated herein for disclosure purposes. The RCA C33016G photomultiplier tube, shown in FIG. 1 of the copending application, comprises a glass envelope having a diameter of 25.4 mm and a length of about 60 mm. As disclosed in the above-identified application, the deleterious effects of shock and vibration can be minimized by using a stem having a plurality of stiff, short support leads connected directly to a plurality of stress isolation eyelets attached to the dynode cage assembly. Flexible nickel wires extend from the stress isolation eyelets to the anode and the dynodes. Unfortunately, in a small photomultiplier tube, such as the C33016G, in which the dynode cage assembly is mounted in proximity to the stem, the dynodes closest to the stem are frequently overheated and damaged when the stem is heat sealed to the envelope. The damage is irreversible and results in the loss of the tube. Properly designed sealing fixtures and the lowest possible sealing temperatures have reduced the number of tubes lost because of overheated dynodes; however, in the highly competitive photomultiplier tube business, it is desirable to further minimize tube losses by eliminating sealing-related dynode damage.
Even in tubes without apparent sealing-related dynode damage, the sealing heat can cause an adverse reaction with the alkali compounds used to form the photocathode and to activate the dynodes. Such tubes exhibit undesirable high temperature instability.